1. Field of the Invention
This invention relates to a microwave plasma apparatus, and more particularly, to a microwave plasma apparatus which is adapted to generate a vacuum ultraviolet light for use in plasma processes such as etching and chemical vapor deposition (CVD), and photoexcited processes such as photo-assisted CVD.
2. Prior Art
FIG. 1 is a cross-sectional view of a microwave plasma apparatus using conventional microwave discharge disclosed in Japanese Patent Public Disclosure No. 131454/86. In the drawing, a microwave transmitting window 13 made of quartz or a ceramic is provided perpendicularly to an electric field generated by a microwave signal which propagates in a waveguide 11 in the direction indicated by an arrow 12. In a discharge space including the window 13 as a surface, there is placed an object to be processed, for example, a wafer 14. Reference numeral 15 designates a stage; 16 a gas lead-in port; and 17 an exhaust port coupled to a known exhaust facility (not shown). An arrow A in the drawing indicates the direction of flow of a gas.
Operation of the above-mentioned apparatus will now be described. A microwave signal transmitted in the waveguide 11 as indicated by the arrow 12 is absorbed by the microwave transmitting window 13 disposed perpendicularly to the direction of electric field of the microwave signal and discharges a gas in the vacuum chamber to generate a plasma. Oxygen gas, for example, if used as a discharge gas, can peel off a resist on the wafer 14.
Since this microwave plasma apparatus is constructed as described above, a plasma tends to be uniformly distributed in the discharge space if a plasma generating area is extended as required by a recent increase in diameters of wafers. More specifically, since the microwave signal is coupled to the plasma so strongly, the microwave signal is rapidly absorbed by the plasma in a direction of travel of the microwave signal, whereby the plasma is inhibited from diffusing. Also, as the plasma distribution in a direction perpendicular to the direction of travel of the microwave signal corresponds to an electric field distribution in the longitudinal direction of the waveguide 11, the plasma tends to be weak in the vicinity of both sides of the discharge space and strong in a central portion of the discharge space, thus being uniformly distributed.
Such a microwave plasma apparatus as mentioned above may be applied to a microwave discharge light source unit in which an ultraviolet light is generated by a generated plasma and employed for photoexcited processes such as photoassisted CVD.
FIG. 2 is a cross-sectional view showing a photoexcited process apparatus using a conventional microwave discharge light source unit shown in Japanese Patent Public Disclosure No. 4762/90. In the drawing, a substrate holder 22 is disposed in a reaction chamber 21, and a substrate 23 is placed thereon. A reaction gas is supplied from a lead-in port 24 into the reaction chamber 21 and exhausted from an exhaust port 25.
One end of a square waveguide 26 is coupled to one side of a circular discharge chamber 28 through a tapered waveguide 27 having one of the E-planes tapered. The other side of the discharge chamber 28 is coupled to a thin waveguide 29 in which a terminal 30 is movably provided. A discharge gas is supplied to the discharge space 31 of the discharge chamber 28 from a lead-in port 32 and exhausted from an exhaust port 33. On the lower surface of the discharge space 31 is provided a dielectric plate 34 made of sapphire or the like to form a light transmissive window for transmitting therethrough an ultraviolet light generated in the discharge space 31. The length of the longitudinal end of the dielectric plate 34, that is, the thickness thereof is substantially the same as the inner diameter of the tapered waveguide 27 and the waveguide 29. Along the lower surface of the dielectric plate 34, a light transmissive microwave reflecting member 35 is provided. Specifically, this reflecting member 35 is located opposite to the discharge space 31 with respect to the dielectric plate 34 and made, for example, of a metal mesh plate which reflects microwaves and transmits light. On the upper side of the discharge space 31 of the discharge chamber 28, there is provided a cooling path 36 in which a coolant liquid is circulated. O-rings are interposed respectively between an end portion of the tapered waveguide 27 and the dielectric plate 34 and between an end portion of the thin waveguide 29 and the dielectric plate 34 to thereby provide a vacuum seal for the discharge space 31. Similarly, a light source apparatus comprised of the discharge chamber 28 is separated from the reaction chamber 21 by another O-ring. In FIG. 2, an arrow A indicates the direction of the generated ultraviolet light; an arrow B the direction of propagation of the microwave; an arrow C the direction of flow of the discharge gas; an arrow D the direction of flow of the reaction gas; an arrow E the direction of the electric field; and an arrow F the direction of flow of the coolant liquid.
Next, operation of the apparatus shown in FIG. 2 will be described. The microwave signal propagated in the square waveguide 26 and its electric field (the arrow E) is gradually intensified by the tapered waveguide 27, and is coupled to the dielectric plate 34. Since the electric field within the waveguide is propagated parallel to the width of the dielectric plate 34, the microwave signal is coupled to the dielectric plate 34 with high efficiency. The microwave signal, while transmitting through the dielectric plate 34, is gradually coupled to the discharge space 31, whereby a gas existing in the discharge space 31 is discharged to cause a light emission. An ultraviolet light thus generated irradiates the substrate 23 placed in the reaction chamber 21 to perform a photoexcited process such as a photo-assisted CVD and an optical etching.
Since the microwave discharge light source apparatus shown in FIG. 2 is constructed such as described above, if a light generating area of the light source is enlarged for irradiating a larger area, the distribution of light generated from the surface of such an enlarged area tends to lack uniformity. FIG. 3 shows results of measuring the distribution of generated light in the direction of travel of the microwave in the microwave signal discharge light source apparatus shown in FIG. 2. In the graph, the abscissa represents the position (cm) in the discharge space in the direction of travel of the microwave signal measured from the microwave supply side, and the ordinate intensity (a.u) of the generated light. As shown in the graph, the intensity of the generated light tends to be strongest at the microwave supply side and becomes gradually weaker at positions further away from the supply side. This is because the microwave signal is attenuated during travel due to strong coupling of microwave with plasma. This tendency becomes more pronounced in relation to larger light generating surfaces, thereby resulting in the problem that luminance distribution becomes nonuniform on a light generating surface.